This invention relates to a voltage-current conversion circuit less in non-linearity distortion.
A voltage-current conversion circuit is to output current which is linear to voltage applied thereto. A simple example of the voltage-current conversion circuit is as shown in FIG. 3. In the circuit, the base of a transistor Q is the input terminal IN of the circuit, and the emitter of the transistor Q is grounded through a load resistor R, and the collector is connected to a positive power source Vcc, thus forming an emitter follower circuit. In the circuit thus arranged, the following Equation (1) is established: ##EQU1## where Vi is the input voltage applied to the input terminal IN, I.sub.E is the Current flowing in the load resistor R; that is, the emitter current, and V.sub.BE is the base-emitter voltage of the transistor Q.
In addition, the following Equation (2) is established: ##EQU2## where I.sub.s is the saturation current due to the diode characteristic of the base-emitter of the transistor Q, q is the electron charge (1.602.times.10.sup.-19 C), K is the Boltzmann's constant (1,38.times.10.sup.-23 J/K), and T is the absolute temperature [.degree. K.].
Hence, the base-emitter voltage V.sub.BE is not linear to the emitter current I.sub.E, and accordingly the characteristic indicated by Equation (1) is the non-linearity characteristic because of the non-linearity characteristic of the base-emitter voltage V.sub.BE. Assuming that the current amplification factor of the transistor Q is represented by h.sub.fe, the input impedance of the circuit is about h.sub.fe times of the load resistance R. However, it is still insufficient in the case where high input impedance is required.
In order to overcome this difficulty, a circuit as shown in FIG. 4 has been provided in which negative feedback is provided by an operational amplifier 40. The non-inversion input terminal of the operational amplifier 40 is employed as the input terminal of the circuit, and the output terminal of the amplifier 40 is connected to the base of the transistor Q, so that the emitter voltage of the transistor Q; i.e., the voltage applied to the load resistor R is fed-back. If it is assumed that the operational amplifier has the ideal characteristic; that is, the input impedance is infinite, the input offset voltage is zero, and the open loop gain is infinite, then the current I.sub.R flowing in the load resistor R is: ##EQU3## Thus, no non-linear distortion is caused which is due to the non-linearity of the base-emitter voltage V.sub.BE, and the input impedance can be made infinite. However, in practice, no operational amplifier ideal in characteristic is available. If, in order to obtain sufficient effects, it were intended to make the characteristic of an operational amplifier as ideal as possible, then the resultant operational amplifier would be unavoidably intricate in internal structure, resulting in an increase in manufacturing cost.
As was described above, the conventional voltage-current conversion circuit is disadvantageous in that the output voltage is distorted because of the non-linear characteristic of the base-emitter of the transistor, and that the distortion cannot be decreased without an intricate circuit such as an operational amplifier, and yet it is impossible to completely eliminate the distortion.